1. Field of the Invention
This invention relates to devices for separating entrained liquid from vapor. In particular, this invention relates to de-entrainment trays used in distillation towers, especially with respect to hydrocarbon processing.
2. Discussion of Related Art
Separation units, such as atmospheric distillation units, vacuum distillation units and product strippers, are major processing units in a refinery. Atmospheric or vacuum distillation units separate crude oil into fractions according to boiling point so downstream processing units, such as hydrogen treating or reforming units, will have feedstocks that meet particular specifications. Crude oil separation is accomplished by fractionating the total crude oil at essentially atmospheric pressure and then feeding a bottoms stream of high boiling hydrocarbons, also known as topped crude, from the atmospheric distillation unit to a second distillation unit operating at a vacuum pressure.
The vacuum distillation unit typically separates the atmospheric unit bottoms into gas oil vapors based on boiling point, including light gas oil, heavy gas oil, vacuum gas oil, and vacuum reduced crude. The vacuum reduced crude is also known as residuum or “resid” and leaves the vacuum distillation unit as a liquid bottoms stream.
In atmospheric or vacuum distillation, lighter hydrocarbons are vaporized and separated from relatively heavier hydrocarbons so that they can be fed downstream for catalytic processing. Although the heavier hydrocarbons do not vaporize, they may be carried into the lighter hydrocarbons due to entrainment. The entrained heavier hydrocarbons are typically contaminated with metals, such as vanadium or nickel, which can poison the downstream catalytic processing, such as hydrotreating, hydrocracking, or fluid catalytic cracking.
In vacuum distillation, bottoms separated from crude oil by an atmospheric distillation unit are fed to a flash zone in the lower portion of the vacuum distillation unit. Various methods of reducing entrainment of residuum from the flash zone have been developed. One conventional assembly uses a bubble-cap tray above the flash zone to cause the vapor to pass through liquid on the bubble-cap tray, thereby allowing vapor to re-entrain liquid droplets. These re-entrained droplets may contain less of the higher boiling components, but their presence in the vapor stream can adversely impact fractionation and downstream processing. In addition, the bubble-cap tray exhibits a pressure drop and thus increases the flash zone pressure required to drive the vapor through the bubble-cap tray, which necessitates a higher flash zone temperature and prevents a deeper cut distillation.
Standard chimney trays have also been used that include a plurality of risers attached to a plate having holes and a hat attached to the top of each riser. Some chimney trays provide two 90 degree direction changes, with the first direction change occurring when a stream from the riser contacts the hat and the second direction change occurring when the stream exits the chimney. An example of a de-entrainment chimney is disclosed in U.S. Pat. No. 4,698,138, the contents of which are incorporated herein by reference. These types of standard chimneys have a lower pressure drop than bubble-caps, but can still allow significant entrainment.
Another example of a de-entrainment tray is disclosed in U.S. Pat. No. 5,972,171, the contents of which are incorporated herein by reference. In this tray assembly, the risers include devices that impart rotational movement to the fluid stream and use a liquid downcomer to transport the de-entrained liquid from the tray back into the flash zone.
Performance of known de-entrainment trays is poor at higher vapor loads, especially at loads having C-factors of 0.35 ft/s (feet per second or fps) or more. The C-factor is a scaling parameter commonly used in distillation that represents the vapor flow that corrects superficial velocity with density. One problem in conventional devices is that separated liquid can become re-entrained. For example, liquid dripping down the inside or outside of a chimney type tray can become re-entrained by the upflowing vapor at high C-factors, which is a measure of vapor load. Also, liquid deposited on the deck of the tray can get re-entrained by the feed vapor at high C loads.
There is a need for a separation device that exhibits improved performance at high vapor loads. It would be beneficial to provide a system in which re-entrainment is avoided at high vapor loads.